DEADLY ACCIDENTS

Some families of people working in the tower were impatient for news about missing relatives.

Gloria Garcia, 53, herself a Pemex worker who was not in the building during the explosion, came to see if she could track down her son, who worked in one of the floors hit.

"I'm calling his phone and he's not answering," Garcia said, weeping as she called repeatedly on her phone. "Nobody knows anything. They won't let me through. I want to see my son whatever state he's in."

Pemex has experienced a number of deadly accidents in recent years and lesser safety problems have been a regular occurrence. In September, 30 people died after an explosion at a Pemex natural gas facility in northern Mexico.

More than 300 were killed when a Pemex natural gas plant on the outskirts of Mexico City exploded in 1984.

Eight years later, about 200 people were killed and 1,500 injured after a series of underground gas explosions in Guadalajara, Mexico's second biggest city. An official investigation found Pemex was partly to blame.

Alberto Islas, a security analyst at consultancy Risk Evaluation, said the explosion at the Pemex offices was another blot against the company's safety record.

"We've seen this time and again at Pemex. They don't have a well-integrated policy," Islas said, noting it would probably take several hours before investigators would be able to determine the cause of the explosion.

Pemex, a symbol of Mexican self-sufficiency since the oil industry was nationalized in 1938, has been held back by inefficiency and corruption and by the burden it shoulders of providing about a third of federal tax revenues.

Pena Nieto has pledged to open up the company to more private investment to improve its performance.